The Straits
Times, Thursday, July 10 2008
A construction worker checking out the completed $226 million Marina Barrage,
a dam built across the Marina Channel to create a freshwater reservoir. Singapore's
first urban freshwater reservoir – at 10,000ha, or one-sixth of Singapore
– will provide desalinated water to meet the country's growing demand.
The barrage will also serve as a flood-control gate and a tourist attraction.
The new reservoir – together with supplies from imported water, Newater
and desalinated water – will help Singapore become self-sufficient in
water by 2061, when its water agreements with Malaysia end. Singapore currently
consumes 1.36 billion litres of water a day, equivalent to 500 Olympic-size
swimming pools.

Actual (August 2008)

The Straits
Times, Monday, August 25 2008
Some 25,000 people ran across the Marina Barrage at part of a half marathon
of 21km yesterday. It was the first time the 300m-long barrage - a dam built
across the Marina Channel to form Marina Reservoir - has been included in the
Safra Singapore Bay Run and Army Half Marathon.

Reported from
Zaobao, this was the first event since the completion of the Barrage. The Marina
Barrage will be open to the public in November.

TECHNICAL
VISIT TO THE MARINA BARRAGE ORGANISED BY THE INSTITUTION OF ENGINEERS, SINGAPORE
(27 FEB 2009)

Barrage view from Pump House to the
East Abutment

Drainage Pump House. View from the
Barrage.

Drainage Pump House. View from the
main entrance.

INTRODUCTION
The Marina Barrage is part of a comprehensive flood control scheme to alleviate
existing flooding in the low-lying areas of the city. With the Barrage and other
flood-alleviation projects, flood-prone areas in Singapore will be further reduced
from the current 150 ha to 85 ha, down from 3200 ha in the 1970s.

The Marina Barrage will
enhance Singapore's water supply in line with Singapore's Four National Taps
water supply strategy to diversify its water sources (The 4 National Taps are:
local catchment, reclaimed water, desalted water and imported water). The Marina
Reservoir will have the largest urban catchment of 10,000 ha among all the reservoirs.
With this Project, about 60 per cent of Singapore will become catchment area.

The Project is unique in
that it is designed to achieve three aims: to act as a tidal barrier for flood
control, to create a new reservoir to augment the water supply and to maintain
a new body of freshwater at constant level in the heart of the city as a major
lifestyle attraction. The aesthetically pleasing water body that is not subjected
to tidal variations provides a new recreational place for sports on the water.

When the Marina Barrage
project is completed by the end of 2007, it will be a prominent landmark in
Marina South. The Project has been carefully designed to blend in well with
the environment, with guidance from Urban Redevelopment Authority (URA)'s Design
Advisory Panel.

HOW DOES THE BARRAGE
WORK?
The barrage, which comprises nine numbers of 26.8-metre-long hydraulically operated
steel crest gates, will be built across the 350m wide Marina Channel to keep
out sea water. Under normal conditions, the steel gates will remain closed to
isolate the reservoir from the sea. During heavy rain, the steel gates will
open as necessary to release excess stormwater to the sea when the tide is low.
However, when it is not possible to do so during high tide, the Drainage Pumping
Station capable of pumping up to 280 cubic metres per second will pump out the
excess stormwater into the sea.

CONSTRUCTION CHALLENGE
The site has two layers of Marine Clay, namely, Upper Marine Clay (~10m thick)
and Lower Marine Clay (~20m thick) spreading over below the fill layer (~10m
thick) of the ground level. The Old Alluvium is generally below 40-50m below.
The temporary double skin sheet pile wall scheme is adopted to form the marine
cofferdam to retain about 11m high hydrostatic and wave pressure to provide
a dry working area within it. Before installation of the marine sheet piles,
the Upper Marine Clay is to be dredged and filled with sand to create sandkey
along marine cofferdam to obtain adequate passive resistance. The 12m wide marine
cofferdam must be robust enough to be self-supported gravity structure. This
is achieved by using two paralleled rows of sheet pile walls with filled sand.
The sheet pile is FSP IV, 42m depth and the cofferdam is filled with sand with
2 rows of tie rods on top to form a gravity wall in the sea . Well-point dewatering
inside the cofferdam is followed and sand berm on the dry side (working area)
is to balance the hydrostatic and wave pressure from sea side. The subsequent
permanent work construction will be carried out after the cofferdam is enclosed.
Total tonnage of the sheet pile is approximately 17,000. The design involves
the contractor’s PE and specialist checker, and PUB’s geotechnical
advisor. The process calls for the interpretation and selection of the appropriate
soil parameters from historical and additional on-site soil investigation. Adequate
instrumentation is installed and weekly monitoring being carried out during
construction.

Driving the marine sheet
piles to the designed 42m deep is never easy. The Contractor has experienced
the initial hard time of experimenting different types of vibro hammers and
piling sequence to gradually pick up the speed of the schedule.

Sandfilling from barge into
the completed double wall sheet piles is another issue. The filling shall be
gradually and evenly distributed so that the pressure to the both side of sheet
pile wall is balanced. Otherwise, the sheet piles would move inward due to one-sided
local pressure. Also, the residual marine silt trapped underneath the sheet
piles shall be totally cleared. The remedial measure for the tilted marine sheet
piles is to dump rocks to the side of sheet pile wall to maintain the balance.

Due to the upper marine
clay underlaid the excavated area, when DPS initial excavation to the formation
level (from ground RL103 to pilecap RL91), the progressive movement recorded
by the inclinometer indicated the soil movement. The record was beyond the alert
level. The affected nearby ground sank by about 800mm. These indicated that
the shear failure curve was formed along the upper marine clay. The excavation
had to be stopped and immediately backfilled to the RL97 to stable the soil.
The locally braced excavation with sheet piles and strutting and the segmental
construction method were introduced. The method is to excavate and cast the
base slab block by block (about 10m x 10m) and in non-continuous pattern so
that the soil disturbance is kept to the minimum. This method though dramatically
reduces the site progress.

Overall, the construction
progress is largely determined by the cofferdam design and the segmental construction
sequence.

Leakage test to the crest
gate and stop log undergoes the learning and some trial-and-error process. In
the end, with cofferdam and add-on temporary dams to separate East Abutment
and Barrage, by using stop logs and locally pumping in water to test the leakage
rate of the stop log and crest gate, two sets of stop logs at one time, the
testing is managed complete.

Underground seepage or piping
beneath permanent structure (energy dissipating basin) at the completed east
part of barrage (first stage of completed barrage) is another issue raised by
the temporary works PE (professional engineer). It was observed that the underground
water was gushing out from the king post embedded in the completed concrete
slab and the narrow gap between the base slab and cofferdam wall, when the enclosed
cofferdam was pumped out of water. The phenomenon is interpreted by the temporary
works PE as the the seepage or piping, which shall be eliminated.

SITE LAYOUT AND
LOGISTICS
- Cranage. Due to the nature of the structure (spreading along a strip and 2
storey only), only enough crawler cranes are used. Crawler cranes access are
along the top of cofferdam and outside discharge channel wall (DPS).
- Access. Traffic access has several stages: excavation, RC super structure,
external works.
- Office, canteen, workers' rest area, M&E storage area.
- Traffic across Marina South (DPS) and Marina East (1st stage of Barrage and
East Abutment): by ferry.

CONTRACTOR'S SITE
ORGANISATION
Generally, this is the two-layers structure. Senior PM, PM, QS, planning and
scheduling, QA/QC and document control, drafters to perform the project control
function, and site managers, engineers, foremen, store keeper to perform operation
function. Of course, there have statutory required safety officer and supervisors.
One of the company's directors is the project director (not station on site)
and the HQ's contract manager helps the contractual matters. Total there are
about 30 site staff and 300 workers (including sub-contractors' workers).